Separation of transparent workpieces

ABSTRACT

A method is provided for preparing transparent workpieces for separation. The method includes generating aligned filament formations extending transversely through the workpiece along an intended breaking line using ultra-short laser pulses.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/711,881 filed May 14, 2015, now allowed, which is a continuation ofInternational Application No. PCT/EP2013/073329 filed Nov. 8, 2013,which claims the benefit of German Application No. 10 2012 110 971.0filed Nov. 14, 2012 and claims the benefit of U.S. ProvisionalApplication 61/726,065 filed Nov. 14, 2012, the entire contents of allof which are incorporated herein by reference.

BACKGROUND 1. Field of the Disclosure

The present invention relates to the preparation for separatingworkpieces and substrates using ultra-short pulsed laser radiation. Inparticular toughened glass or glass ceramics are contemplated as aworkpiece material. The invention also relates to the separation ofworkpieces.

2. Description of Related Art

From WO 2012/006736 A2 it is known that the Kerr effect can be exploitedto cause irreversible damages in glass in form of filaments. Bygenerating a linear array of such damages in glass it is possible toseparate transparent substrates. A filament is formed by an ultra-shortlaser pulse. Due to the Kerr effect, the laser beam experiencesself-focusing in the interior of the glass until the energy density at apoint becomes so high that a plasma is ignited. A plasma explosion iscaused during which the glass undergoes irreversible damage around thisplasma generation location. From there further radiation emanates whichis subject to self-focusing and ends up in a plasma explosion. Thiseffect is repeated several times, depending on the intensity. Energydecreases along the entire thickness of the glass, so the first plasmaspots will have the highest energy and produce the greatest damages.Furthermore, the plasma spots are round, which means that emanatingdefects will occur randomly distributed in all directions.

In glass that exhibits introduced stresses, for example, chemicallytoughened glass, spontaneous self-breakage may occur whereby theprocessing of especially comparatively large glass sheets isconsiderably affected. As a result of breakage the position of the glasssheet changes. Further exact processing is impossible.

Patent document DE 102 13 044 B3 describes a method for cutting ordrilling material. Here, again, the nonlinear optical effect occurringwith high-intensity ultra-short laser pulses is exploited to produce afilament due to the alternating focusing and defocusing of the laserbeam. Accordingly, a filament is a passage of small diameter produced bya high-intensity laser light beam.

Furthermore, document DE 10 2006 042 280 A1 describes a method forprocessing transparent material using a laser. Ultra-short laser pulsesare used to generate both a surface groove on the substrate and one ormore laser-modified regions in the volume of the material. The fractureultimately leading to the separation occurs at the superficial scribingtrace and propagates downwards across the substrate material. If thesurface groove is too flat, the fracture tends to migrate. A generationof breaking edges with consistent high quality is not described.

DE 10 2007 028 042 B2 also discloses a method for laser processing oftransparent materials and describes a use of pulsed laser radiation inthe nanosecond range. The document mentions a range of radiationintensity in which material changes occur without plasma luminescence.

In summary, various processes have been known which allow to modifyregions in the volume of a material by means of ultra-short pulsed laserradiation so as to provide one step of a separation process. However,the separating and breaking which is required for example for dicingsubstrates that have been modified in this manner, has hitherto not beensufficiently accessible to industrial processes. This problem isparticularly acute with substrates comprising toughened glass or glassceramics, as these are prone to uncontrolled breakage due to inherentstresses introduced by the toughening, when processed with ultra-shortpulsed laser radiation.

However, for industrial application exact control is not only requiredfor the generation of a separation line in or on the substrate, but alsofor the separating or breaking in order to produce breaking edges ofconsistent high quality and to ensure stability and safety of theprocess. This is very difficult particularly in case of toughened glass,since the material modifications caused by the laser irradiation canlead to an uncontrolled occurrence and propagation of cracks, so thataccurate control of separation is very difficult.

The following issues are of concern: Cutting/Drilling usingfilamentation: due to the process, formation of the filament occursinhomogeneously: due to the high initial energy density, comparativelylarger plasma volumes are ignited on the entry side of the filamentproducing laser beam than at the subsequent plasma spots deeper in theworkpiece, i.e. the channel of damages in the workpiece (correspondingto the filament formed) will taper. The induced damages (microcracks)will thus be much stronger on the entry side of the laser beam than onthe exit side. Directional strength tests four-point bending reveal asignificant difference in edge strength already with a glass thicknessof 0.7 mm. Spatial geometry of the plasma generation spots: The plasmageneration spots caused by self-focusing have a substantiallyspherically symmetrical shape with spherically symmetrical energydistribution, which causes direction-independent randomly distributedmicrocracks around the plasma volume. As a result, cracks will evenprotrude into the later breaking edge and have a strength-reducingeffect. Spontaneous breakage: During filamentation of brittle materialswith intrinsic stresses, uncontrolled spontaneous breakage of theworkpiece occurs during the process, resulting in an increased rejectionrate. Furthermore, spontaneous breakage causes a change in the positionof the workpiece, so that automated processing is impeded or even madeimpossible.

SUMMARY

A major object of the invention is to improve the quality of the edgesproduced.

The invention permits to improve the separation process for hard andbrittle materials.

Multifilamentation: In contrast to the prior art, the preparation of theseparation by cleaving the workpiece is accomplished not only bygenerating a single tapering filament formation, but by generating aseries of a plurality of consecutive filament formations. Each of thesefilament formations is comparatively narrow and produces a significantlylower number of microcracks transversely to the direction of thefilament formations as compared to the separation process with a singlefilament formation that extends through the workpiece. Due to the lesspronounced tapering of each single filament formation(=filament+microcracks) a better overall geometric accuracy of theprocessing channel is achieved, in combination with a higher edgestrength of the breaking edge when the workpiece has been separated.

The individual filament formations are generated by a picosecond tofemtosecond pulse train which is split and offset in time and introducedinto the workpiece starting from the exit side of the laser beam. Largercutting depths may be realized by multi-pulse sequences at the samelaser power, with a corresponding reduction of the cutting speed.

The spatial geometric shape of the plasma may be influenced by a specialoptic system. The laser radiation beam is generated with an elongatedcross-sectional shape, for example a lancet-shaped, elliptic, ordrop-shaped cross section. In this manner, a controllable preferreddirection of the damages/cracks resulting from the plasma explosions isobtained.

In case the intended breaking line is curved or changes direction, thelaser radiation beam should be controlled in terms of directionalalignment of its cross-sectional shape so that the longitudinalextension of the cross-sectional shape follows the intended separationline of the workpiece.

The generation of filament formations under a protective atmosphere isprimarily an aspect of apparatus configuration, and the manufacturingapparatus is adapted so that the atmosphere surrounding the workpiece tobe processed can be adjusted in a predefined manner.

With a selectively adjusted atmosphere it is possible to inhibit orprevent spontaneous breakage of the workpiece.

The invention also relates to a method for separating a substrate bymeans of focused laser radiation, comprising the steps of: exposing thesubstrate to a protective gas atmosphere; directing ultra-short pulsedlaser radiation onto the substrate, the substrate being transparent inthe wavelength range of the laser radiation; generating a filamentarymaterial modification in depth in a predetermined volume of thesubstrate by the laser irradiation; and breaking the substrate along theseparation line defined by the material modification.

There are two nonlinear optical effects that may be caused by anultra-short pulsed laser radiation in a transparent material, which isthe optical Kerr effect on the one hand, and on the other the defocusingof the laser beam in a plasma bubble. Such effects have already beenknown and will therefore be outlined only briefly.

The Kerr effect refers to a change in the optical property of atransparent material (transparent in the range of wavelengths of thelaser radiation) as a function of the applied or occurring strength ofthe electric field. The laser radiation involves an electric field inthe transparent material whose strength depends on the light intensityof the laser radiation. The electric field causes a change in theoptical characteristics of the irradiated material, including anincrease in refractive index. This in turn leads to the self-focusing ofthe laser radiation.

Due to the self-focusing and the resulting reduction of the irradiatedcross-sectional area, power density per unit area and hence radiationintensity strongly increases and may reach very high values. As aresult, the electric field is further intensified leading to multiphotonionization. Ionization means charge separation in molecules or atoms andplasma formation at focusing spots. Anyway damage is caused in thematerial of the substrate in the focusing spots, which will also bereferred to as a material modification below and manifests itself asplasma bubbles.

In the region of the locally generated plasma bubble defocusing of thelaser beam occurs, which is again followed by a next focusing of thelaser beam. In this manner, a kind of pearl string can be produced inthe material of the substrate, which consists of a series of multipleconsecutively aligned focusing and defocussing regions and which arereferred to as filaments.

Due to the focusing and defocusing, the laser beam may be caused topropagate into the material of the substrate so that the material issubjected to a kind of perforation in depth. This effect has been knownfrom the processing of transparent materials such as glass and is used,for example, to produce a kind of perforation line in the substrate asan intended breaking line or separation line, by moving the substraterelative to the laser radiation.

With a higher laser power, a correspondingly deeper perforation may beachieved, which facilitates separation in the region of the perforation.

However, an application of deeper perforation poses problems inconjunction with pre-stressed glass, that is to say a material whichexhibits increased inherent stress already in the starting state. Theelevated inherent stress may result in spontaneously formation andpropagation of cracks in the perforated regions, as described in WO2012/006736 A2, inter alia. In particular with a comparatively highlaser power, breakage of the substrate may occur already during theprocessing. Therefore, industrial application is considerablycomplicated because the separation process cannot be reliably managedand controlled.

The inventors have found that the spontaneous cracking can be reduced oreven completely prevented if during laser irradiation of the substrate,the workpiece is exposed to a specific atmosphere. For example, crackingis significantly reduced or delayed if a protective gas atmosphere isprevailing during the laser irradiation, which is poor in hydroxyl (OH)ions or even free of OH ions.

By applying a nitrogen atmosphere during the laser irradiation, theperiod of time until spontaneous breaking cracks occurred in the regionof the perforation was sufficiently extended.

In this way it is possible to conclude with the process of laserirradiation of the substrate before spontaneous breaking cracks occur inthe substrate so that the substrate breaks. Accordingly, duringirradiation with ultra-short pulsed laser radiation the substrate isexposed to an atmosphere that is low in OH ions or free of OH ions. Forexample an appropriately gas-tight chamber can be used for this purpose,in which the substrate is accommodated during the laser irradiation.

Particularly good experiences were made with a protective gas atmosphereincluding a water content of less than 0.2 vol %, preferably less than0.1 vol %.

Thus, the laser irradiation enables to generate a filamentary materialmodification in depth similar to a perforation in a predetermined volumeof the substrate. The predetermined volume of the substrate refers tothe predefined separation area along which the perforation is to beproduced and which therefore defines the later separation area orseparation line on the surface of the substrate.

Particularly advantageously, the substrate or the laser beam may bemoved relative to one another in order to enable the perforation in thedepth of the substrate along the separation line in this way. Typically,the substrate may be moved, for example by means of an X-Y axesadjustment assembly which allows two-dimensional displacement of thesubstrate with a constant distance to the laser radiation. Thisarrangement may additionally be combined with a movable Z-axisarrangement in order to enable adjustment of the distance between thesubstrate and the laser beam.

The substrate material is optically transparent at least in the range ofwavelengths of the laser radiation, with an optical transmittance inthis range of at least 80%/m, preferably at least 85%/cm, and mostpreferably at least 90%/cm. Thus, the laser radiation can penetrate intothe material.

The substrate may comprise materials selected from a group comprisingglass, sapphire, and diamond. Surprisingly, the laser irradiationaccording to the invention can even be employed for toughened glass asknown for display applications, for example, and also with sheet glass.Furthermore surprisingly, glass ceramic materials can also be processed.

The perforation in depth produced in the substrate along a path extendsalong the separation line at which a separation of the substrate isintended to occur. The separation line may be a rectilinear line, butalso a non-straight or curved line. For example, the separation line mayhave very small radii to enable a separation of the material similar toa drilled hole.

The perforation may extend from the surface of the substrate verticallyinto the depth. However, it may likewise be produced at a certain anglewith respect to the surface, for example to produce oblique separationedges at the substrate. In this case the laser beam is not directedperpendicularly but rather at a predetermined angle to the surface ofthe substrate.

Subsequently, separation of the material is accomplished along theperforated separation line. For this purpose it is necessary that thematerial modification which is generated by the perforation and isequivalent to a damage of the material reaches a particular extent inorder to enable separation with a specific surface quality of theseparation edge and strength of the separated substrate.

It has been found that the separation at the separation line is welldone when the perforation extends into a depth corresponding to at least40% of the material thickness of the substrate, preferably at least 50%,and most preferably at least 55%. In this case the filaments should havea spacing to each other ranging from about 200 μm to 800 μm. A filamentmay have a cross section in a range from about 15 μm to 250 μm.

In this manner, a sufficiently extensive pre-damage of the substratematerial may be achieved along the separation line resulting in goodseparability. When separating the material along the separation line,separation edges are formed with a surprisingly good quality of the soproduced separation edges.

In the region of perforation, the separation edges typically exhibit apattern of adjacent parallel filaments, and in the underlying region ofthe separation edge a rather conchoidal fracture pattern. Roughnessvalues obtained for the separation edge were in a range of Ra<100 μm.

The separation edges produced moreover exhibit high level edge strength.This was determined by a 4-point bending test. The average strengthvalue achieved was at least 120 MPa in case of toughened glass.

The laser source is selected according to the range of wavelengths inwhich the substrate is transparent. The wavelength range of the emittedradiation is within the transmission range of the substrate.

The laser beam may be spatially focused with a Gaussian intensitydistribution in order to achieve a sufficiently high intensity. Thefirst focal point is located within the substrate, that is to say in thesubstrate volume. When the laser pulse hits this point in the volume ofthe substrate, a plasma may be generated and hence a materialmodification may be produced there. With the subsequent defocusing andfurther focusing effects, the filamentary perforation comprising aplurality of focusing points can be produced in the substrate volume.

The focusing spots in transparent glass, for example, often have aspherical approximately symmetric shape. However, by means of a specialoptic system it was possible to generate focusing points ofnon-spherical spatial shape in toughened glass. For example, elliptic,lancet-shaped, or drop-shaped focusing spots were generated. Such shapesof the focusing spots promote a formation of cracks in particular fromone focusing spot to the next due to a better propagation of the crack,thus also improving the quality of the separation edge that can beachieved.

A suitable laser source according to the present invention operates witha repetition rate between 10 kHz and 120 kHz, preferably between 30 kHzand 110 kHz, and most preferably between 35 kHz and 105 kHz.

An appropriate pulse duration of a laser pulse is in a range below 100picoseconds, preferably less than 10 picoseconds, and most preferablyless than 1 picosecond. Particularly favorably, the laser source isoperated at a power in a range from about 7 to 12 watts.

With such a laser radiation, very good results were obtained for thegeneration of separation lines on toughened glass and perforated indepth.

An increase of the laser power may lead to a greater tendency tospontaneous formation of cracks, in particular in toughened glass,whereas in non-toughened glass it is possible to achieve a more denseperforation which in turn improves separability.

The invention further relates to an apparatus for separating asubstrate, in particular toughened glass or glass ceramic, by means offocused laser radiation, comprising: a gas-tight chamber foraccommodating the substrate; an ultra-short pulsed laser light source;means for displacing the substrate and/or the laser light sourcerelative to each other, while: the substrate is exposed to a protectivegas atmosphere; ultra-short pulsed laser radiation is directed onto thesubstrate, which substrate is transparent in the wavelength range of thelaser radiation; by the laser irradiation, a filamentary materialmodification is generated in a predetermined volume and extending intothe depth of the substrate; separation is accomplished along theseparation line defined by the material modification.

The invention further relates to an article of toughened glass or glassceramic that has been processed at least on one side by a methodaccording to the invention.

In a modification of the method for separating a substrate, inparticular toughened glass or glass ceramic, by means of focused laserradiation, the substrate is exposed to a second atmosphere following theperforation of the substrate along the later separation line. Thissecond atmosphere is different from the protective gas atmosphere whichaccordingly represents the first atmosphere, in its content of hydroxyl(OH) ions. The second atmosphere has a higher content of OH ions thanthe first atmosphere.

The inventors have found that an increased content of OH ions canpromote the separation or cleaving of the workpiece along the perforatedseparation line. By exposition to an atmosphere enriched in OH ions,such as wet steam, a formation of cracks may be promoted and thuscontrolled. In this way, the process step of cleaving for accomplishingmaterial separation can be selectively influenced, so that easyindustrial applicability is provided. In particular it is possible toprevent spontaneous breaking cracks from occurring.

Particularly good experiences have been made with a second atmospherehaving an OH ions content of at least 1.4 vol %, preferably at least 2vol %.

Therefore, particularly advantageously, the chamber for accommodatingthe substrate may be gas-tight so that a first atmosphere poor in OHions is easily established. Furthermore particularly advantageously,this chamber may as well be adapted for exposing the substrate to an OHion enriched atmosphere. But it is likewise possible that the apparatuscomprises two separate chambers which are adapted accordingly forapplying the first or the second atmosphere.

Further details of the invention will become apparent from thedescription of the illustrated exemplary embodiments and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described withreference to the drawings, wherein:

FIG. 1 shows a laser processing device while processing a workpiece on aworkpiece table;

FIG. 2 shows an enlarged detail of FIG. 1;

FIG. 3 shows an elliptical beam cross section of the laser processingdevice;

FIG. 4 shows lancet-shaped beam cross sections; and

FIG. 5 shows drop-shaped beam cross sections.

DETAILED DESCRIPTION

FIG. 1 shows a laser processing device 1 above a workpiece 2 resting ona workpiece table 3. The laser processing device comprises anultra-short pulsed laser 10 and a focusing optic system 11 to provide afocused radiation beam 12 having a focal point 13 near the upper surfaceof the workpiece 2. A cut line or breaking line 20 is indicated onworkpiece 2, along which line the workpiece is to be separated orcleaved. Provisions are made so that the focus 13 can be displaced alongthis line 20, which is facilitated by adjusting the table in the twocoordinate directions 21, 22. Very small adjustment increments are used.

The ultra-short pulsed laser 10 is able to deliver laser pulse trains intwo or more successive periods. The wavelength of the radiation ischosen so as to be in a range for which the workpiece 2 is transparent.The energy of the laser pulses is dimensioned so that in each case arespective line-shaped damage formation 14 is formed transversely to thesurface of workpiece 2. By displacing the focusing optic system 11 alongthe intended breaking line 20, a series of line-shaped damage formations14 is generated in the workpiece 2, which virtually define the intendedbreaking face. The invention relates to the generation of this series ofline-shaped damage formations 14 along the line 20.

FIG. 2 schematically shows a damage formation 14 extending transverselythrough the workpiece 2. In the illustrated exemplary embodiment, thedamage formation 14 comprises three filament formations 4, 5, and 6aligned along a straight line. Each of these filament formations isgenerated by an ultra-short pulsed laser pulse train. In case offilament formation 4, a plasma spot 41 is generated due to self-focusingof the laser beam 12, at which plasma spot the material of the workpiecetransitions into the plasma phase, which is accompanied by emittedradiation 42 which due to self-focusing leads to a further plasma spot43, and the process continues until the energy of the laser pulse isexhausted. A virtual plasma explosion takes place at plasma generationspots 41, 43, 45, due to thermal expansion, which causes cracks formingmainly along a gap that is created into the interior of the workpiece,which is desirable, but also transversely to this channel, as indicatedby cracks 46. These transverse cracks 46 are undesirable and areintended to be kept as small as possible with the invention.

For this purpose, the damage formation 14 is created in several stages.This is achieved by emitting the laser pulses in two or more successiveperiods. The energy of the laser pulses during one period is chosen tobe so small that only a few small plasma explosion spots are producedduring one emission period. In this way, a formation of detrimentallateral cracks 46 is significantly reduced. Along filament formation 4gaps and cleaving cracks are formed which predefine the later fracturein the workpiece.

In a second laser pulse period, filament formation 5 is generatedsimilarly as filament formation 4. Plasma bubbles 51, 53, 55 anddefocusing-focusing spots 52, 54 are produced; similarly as with plasmabubbles 61, 63 and defocusing-focusing spots 62 during the third laserpulse period. The greater depth approach is successful due to thepreviously formed gaps and cleaving cracks in the direction of thebreaking point which virtually presents a guiding channel for the secondand subsequent laser pulse periods. The number of successive laser pulseperiods is determined according to the thickness of workpiece 2.

To promote a formation of gaps in the direction of breaking face 20 itis useful to choose cross-sectional shapes of the laser beam which areelongated or have a larger dimension in the intended breaking direction.Such cross-sectional shapes are illustrated in FIGS. 3, 4, and 5. Theelliptical cross-sectional shape may be obtained based on an originallycircular cross-sectional shape of the laser beam by combiningcylindrical lenses. The lancet-like shape of FIG. 4 and the drop shapeof FIG. 5 of the beam cross section can be obtained by special lenses.These shapes promote cleaving cracks and gaps in the direction of theintended breaking line 20.

If, as illustrated, the breaking line is desired to be curved, the laserradiation beam has to be controlled accordingly so that the largercross-sectional dimension is continuously aligned along the intendedbreaking direction.

When processing brittle material, there is a risk that the workpiecespontaneously breaks when the damage formations along the intendedbreaking line 20 have not yet been all completed. In order to minimizeor completely avoid this risk, the processing is performed in a“neutral” atmosphere, such as under nitrogen. In this manner, preparedworkpieces are obtained which are prepared for being separated orcleaved. The final separation or cleaving is then performed bysubjecting the workpiece to a mechanical tension, and under water vaporor in another atmosphere containing hydroxyl (OH) groups.

It should be noted that the finer the fracture pattern in the separationplane is desired to be obtained, the smaller the spacings are chosenbetween the locations of damage formations 14 along line 20. Thespacings are of the same order of magnitude as the diameter of thedamage formations.

What is claimed is:
 1. A method for separating a workpiece by focusedlaser radiation, comprising: exposing the workpiece to a firstatmosphere including protective gas; directing ultra-short pulsed laserradiation onto the workpiece, the workpiece being transparent in a rangeof wavelengths of the laser radiation to cause a filamentary materialmodification in depth in the workpiece; moving the workpiece and/orlaser radiation with respect to one another to define a separation areain the workpiece; exposing, after the laser irradiation, the workpieceto a second atmosphere including a content of hydroxyl (OH) ions that ishigher than that of the protective gas atmosphere; breaking theworkpiece along the separation area defined by the materialmodification.
 2. The method as claimed in claim 1, wherein the workpiececomprises toughened glass or glass ceramics.
 3. An apparatus forseparating glass or glass ceramics by focused laser radiation,comprising: a workpiece chamber for accommodating the glass or glassceramics; a workpiece feeder that feed the glass or glass ceramics intothe workpiece chamber; an ultra-short pulsed laser light source thatgenerates a filamentary material modification in depth in the glass orglass ceramics by laser irradiation; a displacing device that moves theworkpiece and/or the laser light source relative to each another; wetsteam feed device that feeds a gas stream into the workpiece chamber;and a separating device that separates the workpiece along a separationline defined by the material modification.